Surge absorber

ABSTRACT

In contrast to a conventional surge absorber in which external terminals (4a), (4b) are led from disshaped electrode plates (3a), (3b) soldered to the surfaces of electrodes (2) on a varistor element (1), a surge absorber of the present invention has a varistor element (5), electrodes (6) provided on both sides of the varistor element (5) and electrode plates (7a), (7b) soldered to the surfaces of the electrodes (6), each electrode plates (7a), (7b) having at least one of substantially radial slots and an external terminal (8a), (8b) extended substantially from the center thereof. The surge absorber according to the present invention enables the electrode plates (7a), (7b) to be held in uniform contact with the electrodes (6) during soldering by virtue of the fact that the external terminals (8a), (8b) are led from the central regions thereof, so that the soldering can be conducted effected stably. In addition, since the external terminals (8a), (8b) and led from the central regions of the electrode plates, the electric current in the varistor element is uniformalized to provide a greater surge proof current capacity.

TECHNICAL FIELD

The present invention relates to a surge absorber for protectingelectronic devices from abnormal over voltages such as a switching surgevoltage, a lighting surge voltage, and so forth.

BACKGROUND ART

Current diversification of functions of electronic devices have putelectronics using semiconductors into practical use in various fieldsincluding household devices, data processing devices, communicationsystems and industrial devices. Semiconductors such as ICs, LSIs,thyristors and so forth used in these electronics have excellentperformance but are very sensitive to abnormal voltage which may beapplied by electrostatic discharges, lightning and so forth, so thattroubles such as malfunction or breakdown of the electronic devices tendto be caused when abnormal voltage is exerted thereto. It is therefore amatter of great significance to take a suitable measure against surgevoltages from the view point of security and an improvement of theelectronic devices.

FIGS. 6(a), 6(b) and 6(c) show an example of a known surge absorber.Referring to these Figures, a reference numeral 1 denotes a tabularvaristor element usually made of a semiconductor ceramics mainlycomposed of zinc oxide, strontium titanate or the like, while 2 denoteselectrodes which are provided on the corresponding portions of the frontand rear sides of the varistor element 1 by, for example, firing ofsilver paste. The electrode 2 on the rear side of the varistor element 1is not shown in these Figures. Numerals 3a and 3b show metallicelectrode plates made of materials which have a high degree ofelectrical conductivity such as copper or brass. The electrode plates 3aand 3b are connected to the electrodes 2 by soldering of solder paste. Areference numeral 4a designates an external terminal which is led from aportion of the electrode plate 3a. Usually, the external terminal 4a ismade of the same material as the electrode plate 3a. This applies alsoto another external terminal 4b provided on the rear side. The ends ofthe external terminals 4a and 4b are connected to an electric circuit bysoldering or by means of bolts. Practically, the surge absorber isresin-coated or resin-molded such that only the ends of the externalterminals are exposed, though not shown in FIGS. 6(a) to 6(c).

The operation of this known surge absorber will be describedhereinunder.

The external terminals 4a and 4b are connected between the power supplylines, signal lines or grounding lines of the device to be protected, soas to absorb any abnormal voltage which has been introduced into theselines due to, for example, electrostatic discharge, lightning surge, andso forth. The surge current produced by the abnormal voltage flows fromthe external terminal 4a on the front side of the surge absorber to theexternal terminal 4b on the rear side thereof, through electrode plate3a, electrode 2, varistor element 1, and the corresponding portions onthe rear side of the surge absorber so that the voltage suppressed to asafe level is applied to the device to be protected.

This known arrangement, however, suffers from a disadvantage inconnection with the production. Namely, in producing the surge absorber,external terminals 4a and 4b are held by means of jigs so as to supportthe varistor element 1 therebetween by spring action of the externalterminals 4a, 4b during soldering of the electrodes 2, 2 and theelectrode plates 3a, 3b together. In such a case, the varistor element 1is supported solely by the external terminals 4a and 4b only at the freeends thereof so that only a slight variation in the thickness of thevaristor element 1 causes uniform contact between the electrode plates3a, 3b and the electrodes 2 on the element 1 to be not ensured,resulting in that the soldering cannot be done uniformly.

Even if the electrode plates 3a, 3b are held in uniform contact with theelectrodes 2 on both sides of the element 1, flux voids tend to remainbetween the electrode plates 3a, 3b and the electrodes 2, thus making itdifficult to ensure uniformity of the soldering.

These unfavorable factors undesirably impair performance in regard towithstanding surge current capacity, which is one of the most criticalrequirements for a surge absorber, resulting in a lowered reliability ofthe surge absorber.

The soldering of the electrode plates 3a, 3b to the electrodes 2 isusually conducted by printing paste solder on the surfaces of theelectrodes 2, drying the paste solder and pressing the electrodes plates3a, 3b onto the electrodes 2 under application of heat. Such a method,however, is very expensive.

DISCLOSURE OF THE INVENTION

In order to overcome the above-described problems of the prior art, thepresent invention provides a surge absorber in which the electrodeplates to be soldered to the electrodes on both sides of a tabularvaristor have a specific structure. More specifically, a slot is formedin the electrode plate substantially in the radial direction thereof soas to allow an external terminal to be led substantially from the centerof the electrode plate. In addition, each electrode plate to be solderedto the electrode of the varistor has the form of a plurality of lineswhich extend from a single point or a line. In other words, theelectrode on each side of the varistor is soldered thereon with anelectrode plate having line electrodes which extend radially fromapproximately the center region of the associated electrode or whichextend on both sides from a center line electrode that extendsdiametrically of the electrode.

With this arrangement of the invention, a uniform contact and, hence,uniform soldering between the electrode and the electrode plate can beachieved regardless of any slight variation in the thickness of thevaristor element, partly because a support is given by the lead-outportion of the external terminal lead of which is connected to thecenter portion of the electrode plate, and partly because a certaindegree of flexibility is provided at the portion from which the externalelectrode plate is led, by virtue of the formation of the substantiallyradial slot in the electrode plate. In addition, since the electrodeplates uniformly make contact with the electrode, the electrodes canhold the varistor element with a greater level of stability during thesoldering.

With this arrangement, since each electrode plate soldered on theelectrode on the varistor has a plurality of line electrodes, solderingcan be achieved uniformly without allowing flux and air voids to remainbetween the electrode plates and the electrodes. Further thisarrangement also reduces the production cost remarkably because thesoldering can be effected by a solder dip method without the aid ofsolder paste.

Since a satisfactory contact condition between the electrode on thevaristor and each electrode plate is achieved, the surge currentabsorption capacity is enhanced in comparison with that of aconventional device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 are illustrations of different embodiments of a surgeabsorber in accordance with the present invention in which sections (a),(b) and (c) are side elevational views, front elevational views andperspective views, respectively.

FIGS. 6(a), 6(b) and 6(c) are a side elevational view, a frontelevational view and a perspective view of a conventional surgeabsorber.

THE BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a surge absorber wherein sections (a), (b) and (c) of thisFigure are a side elevational view, a front elevational view and aperspective view of this embodiment.

Referring to FIG. 1, a reference numeral 5 denotes a varistor elementmade of a material which is mainly composed of zinc oxide, 6 electrodesformed on the front and rear surfaces (the rear surface is not shown) ofthe varistor element 5 and 8a and 8b external terminals which are led inthe same direction and which are bent at their one end inwardly asviewed in the thicknesswise direction of the varistor element 5. The oneends of the external terminals may be provided with notches or holes forexternal wiring. These elements correspond to the varistor element 1,electrodes 2, and external terminals 4a and 4b of the conventional surgeabsorber. Numerals 7a and 7b denote electrode plates which are providedwith slots 9 which extend substantially in a radial direction and whichare soldered to the electrodes 6 by solder paste. The electrode plates7a, 7b are made of a material having a high level of electricalconductivity, e.g., copper, a copper alloy or iron, and is plated ontheir outer surface with solder. The external terminals 8a, 8b and theelectrode plates 7a, 7b are formed from a metal sheet by press work. Thewidths of them are adjusted at the connecting portions so as to providesufficient flexibility at these connecting portions. The externalterminals 8a and 8b are led substantially from the central portions ofthe electrode plates 7a, 7b.

Although not illustrated in FIG. 1, the surge absorber in accordancewith the present invention is in most cases coated or molded with aninsulating resin such that only ends of the external terminals 8a and 8bare exposed. Such coating or molding may be omitted if an insulatinglayer of a glass, resin or the like is formed on the outer peripheralportion of the varistor element 5. The external terminals 8a and 8b inthe device of the present invention has a double function: namely afunction as electric terminals and a terminals and a function asstructural members which support the body of the surge absorber.

The operation of the surge absorber of FIG. 1 will be explainedhereinafter. When a surge voltage is applied to the surge absorber, asurge current flows through the varistor element 5 and the surge isabsorbed substantially in the same manner as that in the conventionalsurge absorber. In the FIG. 1 surge absorber, however, since theelectrode plates 7a and 7b which are connected to the electrodes 6 haveexternal terminals which are led from the substantially central portionsof the electrode plates 7a and 7b, the electrode plates 7a and 7b areheld in uniform contact with the electrodes 6 during soldering so thatthe electrode plates 7a and 7b are uniformly soldered and connected tothe electrodes 6, thus eliminating degradation of performance such asthe withstanding surge current capacity. In addition, since the surgecurrent enters the varistor element 5 substantially at the centralportion of the varistor element 5, a uniform electric currentdistribution is obtained throughout the varistor element 5. This alsocontributes to an improvement in the withstanding surge current capacityof the surge absorber.

In addition, since the external terminals 8a and 8b are bent at theirouter ends inwardly as viewed in the direction thicknesswise of thevaristor element, the space required for connection of these terminalscan be saved. In addition, the bent ends of the external terminal 8a and8b enables the surge absorber to stand by itself. Practically, the wholeportion of the surge absorber except for the ends of the externalterminals 8a and 8b is coated or molded with a resin so that the surgeabsorber can have superior abilities of weather-resistance andelectrical insulation.

A first embodiment of the invention will be described with reference toFIG. 2. The sections (a), (b) and (c) of this Figure are a sideelevational view, a front elevational view and a perspective view of thesecond embodiment.

Referring to FIG. 2, a reference numeral 5 denotes a varistor elementmade of a material preferably composed of, for example, zinc oxide, 6denotes electrodes formed on the front and rear sides of the varistorelement 5, and 8a and 8b denote external terminals. These portionscorrespond to the varistor element 1, electrodes 2 and externalterminals 4a, 4b of the conventional surge absorber. Numerals 7a and 7bdenote electrode plates each having a plurality of line electrodes 102,106 which extend radially outwardly from a central region of theassociated electrode 6 as shown in FIG. 2. These electrode plates aresoldered to corresponding electrodes 6 by dip-soldering. The electrodeplates 7a, 7b having the line electrodes 10a and 10b are made of amaterial which have high electrical conductivity, e.g., copper, a copperalloy or iron. Each line electrode 10a, 10b has a width of 0.5 to 2.5mm, and the external electrodes 8a and 8b are connected to the centralregions from which the line electrodes 10a and 10b extending radiallyoutwardly therefrom. An outer resin coating is omitted from the Figure.

The operation of the surge absorber having the described constructionwill be explained hereinunder.

As in the case of a conventional surge absorber, a surge current flowsthrough the varistor element 5 when a surge voltage is applied theretoand the surge is absorbed substantially in the same manner as the firstembodiment. In this embodiment, since the connection to the electrodes 6is achieved through a plurality of radial line electrodes 10a, 10b, fluxand air voids generated in the course of soldering can easily escapethrough the gap between adjacent line electrodes without being trappedbetween the line electrodes and the electrodes 6, whereby the lineelectrodes 10a, 10b are uniformly soldered to the electrodes 6, thusenhancing the performance in regard to the withstanding surge currentcapability The use of the radial line electrodes 10a, 10b enables theuse of a solder-dip method which is inexpensive to carry out. Namely,the external terminals 8a and 8b are suitably held such that thevaristor element 5 is pinched between the line electrodes 10a, 10b andthese parts are dipped in a solder bath whereby the soldering iseffected without requiring printing of solder, drying and heating whichhave to be employed in ordinary paste soldering methods. Further, thesurge absorber may be preferably coated or molded with a resin, exceptat the free end parts of the external terminals 8a, 8b so as to provideexcellent weather resistance and insulation.

A second embodiment of the present invention will be described withreference to FIG. 3. Sections (a), (b) and (c) of this Figure are a sideelevational view, a front elevational view and a perspective view of thethird embodiment. This third embodiment is different from the secondembodiment only in the form of the line electrodes. Namely, in contrastto the line electrodes 10a, 10b which radially extend from the centralregions of electrodes 6 the line electrodes 11a, 11b in this embodimentextend on both sides from a single center electrode extendingdiametrically of the electrode 6. This surge absorber operatessubstantially in the same manner as the embodiment shown in FIG. 2.

A third embodiment of the present invention will be described withreference to FIG. 4. Sections (a), (b) and (c) of this Figure are a sideelevational view, a front elevational view and a perspective view of thefourth embodiment. In this embodiment, the line electrodes are wholly orpartially connected at their outer ends. Namely, line electrodes 12a and12b which are similar to those of FIG. 2 are connected together at theirouter ends by means of a common line electrode 13. Although in theembodiment shown in FIG. 4 all the line 12a or 12b are connectedtogether at their outer ends, this is only illustrative and some ofthese line electrodes may not be connected. This surge absorber operatesin the same manner as that shown in FIG. 2. However, entanglement of theends of the line electrodes 12a and 12b is avoided by the provision ofthe line electrodes 13 so that the efficiency of the assembly work isimproved and the force with which the varistor element is held is alsoincreased advantageously.

A fourth embodiment of the present invention will be describedhereinunder with reference to FIG. 5. Sections (a), (b) and (c) of thisFigure are a side elevational view, a front elevational view and aperspective view of the fourth embodiment. The fifth embodiment isdiscriminated from the fourth embodiment in that the external terminals8a and 8b are lead from the substantially central regions from which theline electrodes 12a and 12b radially extend. A reference numeral 14denotes slots from which the external terminals 8a, 8b are led outsimilar to that shown in FIG. 1.

This surge absorber operates substantially in the same manner as thatshown in FIG. 4. The fourth embodiment, however, offers an additionaladvantage in that the electrical current is uniformly distributedthroughout the varistor element as in the case of the FIG. 1 device.

INDUSTRIAL APPLICABILITY

As has been described, electrode plates each having a substantiallyradial slot and having an external terminal which is led substantiallyfrom the central portion thereof are soldered to the surfaces ofelectrodes which are provided on both sides of a tabular varistorelement. Alternatively, each electrode plate is constructed in the formof a plurality of line electrodes which extend from a single point or asingle line, and an external terminal is led from one of these lineelectrodes. In other words, each of the electrodes provided on bothsides of a tabular varistor has soldered thereon an electrode platehaving a plurality of line electrodes which extend radially or whichextend on both sides from a center electrode that extends diametricallyof the electrode on the varistor. Thus, according to the invention, theelectrode plates are held in uniform contact with the electrodes duringsoldering between the electrode plates and the electrodes on both sidesof the varistor element, so that soldering can be effected uniformlywithout allowing flux and air voids to remain between the electrodeplates and the electrodes, whereby a surge absorber having an improvedwithstanding surge current capability and high degree of reliability canbe obtained.

In particular, in the embodiment in which the external terminal is ledsubstantially from the center of the electrode plate, the surge currentflows into the varistor element from the central region of the varistorelement so that the current is uniformly distributed throughout thevaristor element, whereby the effect of improvement in the withstandingsurge current capability is enhanced. In addition, the stability ofholding of the varistor element during soldering is increased so thatthe production process becomes applicable also to large-size varistorelement.

Further, since the electrode plate has the form of a plurality of lineelectrodes, soldering can be effected by a solder dipping method withoutmaking use of solder paste, so that the cost can be reduced remarkably.In this embodiment, entanglement of the line electrodes at their freeends can be avoided by providing a line electrode which connect at leastsome of the line electrodes. This arrangement also contributes toimprovement in the efficiency of the assembly work and ensures that thevaristor element is held with a higher stability.

In addition, in the arrangement in which the ends of the externalterminal are bent inwardly as viewed in the direction of thickness ofthe varistor element, the space for connection of the electricalterminals is conserved advantageously.

Furthermore, a higher degree of weather resistance and insulation powercan be obtained by coating or molding the surge absorber such that onlythe ends of the external terminals are exposed.

What is claimed is:
 1. A surge absorber comprising a tabular varistorelement, electrodes provided on both sides of said varistor element,electrode plates soldered to the respective electrodes, each saidelectrode plate having a plurality of line electrodes which extend fromapproximately a center region of said associated electrode on saidvaristor and an external terminal led from one part of said eachelectrode plate.
 2. A surge absorber according to claim 1, wherein endsof said line electrodes are wholly or partially connected through a lineelectrode.
 3. A surge absorber according to claim 1, wherein saidexternal terminals are led substantially in a same direction and outerends of said external terminals are bent inwardly as viewed in athickness dimension of said varistor element.
 4. A surge absorberaccording to claim 1, wherein said surge absorber is coated or moldedwith an insulating resin such that said external terminal is exposedonly at its ends.
 5. A surge absorber comprising a tabular varistorelement, electrodes provided on both sides of said varistor element,electrode plates soldered to said respective electrodes, each saidelectrode plate having a center line electrode which extendsdiametrically relative to said associated electrode on said varistor, aplurality of line electrodes which extend on both sides from said centerline electrode, and an external terminal led form one part of said eachelectrode plate.